String for use in racket

ABSTRACT

A method of manufacturing a string  10  for use in a racket includes: (A) preparing an untreated string; and (B) irradiating the untreated string with radioactive rays. Preferably, the untreated string is irradiated with gamma rays. A preferable irradiation amount of the radioactive rays is greater than or equal to 250 kGy but less than or equal to 2000 kGy. Preferably, a material of the untreated string is polyester. Alternatively, the material of the untreated string may be nylon.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Japanese Patent Application No. 2020-083299, filed on May 11, 2020, the entire disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to the strings of, for example, tennis rackets, badminton rackets, and squash rackets.

Description of the Related Art

A tennis racket includes a frame and a string. In the past, natural gut was primarily used as the string. Natural gut is made of sheep intestine. Natural gut is inferior in terms of durability. In addition, natural gut is expensive. However, natural gut has an excellent hitting feeling.

In recent years, mainstream strings are synthetic resin strings. Synthetic resin strings have excellent durability. A string made of a resin composition whose base material is polyester or nylon is widely used. Japanese Laid-Open Patent Application Publication No. 2009-226107 discloses a string in which polyester is used.

When a player hits a tennis ball with a tennis racket including a polyester string, the hitting feeling is hard. The hitting feeling of nylon strings is better than the hitting feeling of polyester strings. However, the hitting feeling of nylon strings is still harder than the hitting feeling of natural gut.

An object of the present invention is to provide a string for use in a racket, the string having an excellent hitting feeling-durability balance.

SUMMARY OF THE INVENTION

A method of manufacturing a string for use in a racket according to the present invention includes: (A) preparing an untreated string; and (B) irradiating the untreated string with radioactive rays.

A string for use in a racket according to the present invention has an excellent hitting feeling and excellent durability.

Preferably, the step (B) includes irradiating the untreated string with gamma rays. Preferably, the step (B) includes irradiating the untreated string with the radioactive rays in an irradiation amount of greater than or equal to 250 kGy but less than or equal to 2000 kGy.

Preferably, a material of the untreated string prepared in the step (A) is a resin composition whose base material is a thermoplastic resin. Preferably, the thermoplastic resin is polyester or nylon.

Another aspect of the present invention is directed to a string for use in a racket, and an elongation rate P1 of the string when the string is tensioned to a stress of 217 MPa is higher than or equal to 9.25% but lower than or equal to 18.6%. Preferably, a material of the string is a resin composition whose base material is a thermoplastic resin. Preferably, the thermoplastic resin is polyester.

Yet another aspect of the present invention is directed to a string for use in a racket, and an elongation rate P2 of the string when the string is tensioned to a stress of 221 MPa is higher than or equal to 19.3% but lower than or equal to 22.0%. Preferably, a material of the string is a resin composition whose base material is a thermoplastic resin. Preferably, the thermoplastic resin is nylon.

Yet another aspect of the present invention is directed to a racket including a frame and a string, and an elongation rate P1 of the string when the string is tensioned to a stress of 217 MPa is higher than or equal to 9.25% but lower than or equal to 18.6%.

Yet another aspect of the present invention is directed to a racket including a frame and a string, and an elongation rate P2 of the string when the string is tensioned to a stress of 221 MPa is higher than or equal to 19.3% but lower than or equal to 22.0%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a tennis racket according to one embodiment of the present invention.

FIG. 2 is an enlarged perspective view of a part of a string of the tennis racket of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention is described in detail based on preferred embodiments with appropriate reference to the accompanying drawings.

FIG. 1 shows a tennis racket 2. The tennis racket 2 includes a frame 4, a grip 6, an end cap 8, and a string 10. The tennis racket 2 can be used in regulation-ball tennis.

The frame 4 includes a head 12, two throats 14, and a shaft 16. The head 12 forms the contour of a face 18. The front shape of the head 12 is substantially an ellipse. The major axis direction of the ellipse coincides with the axial direction Y of the tennis racket 2. The minor axis direction of the ellipse coincides with the width direction X of the tennis racket 2. One end of each throat 14 is continuous with the head 12. At the vicinity of the other end thereof, the throat 14 merges with the other throat 14. The throats 14 extend from the head 12 to the shaft 16. The shaft 16 extends from the location where the two throats 14 merge together. The shaft 16 is formed so as to be continuous with, and integrated with, the throats 14. A portion of the head 12, the portion being positioned between the two throats 14, is a yoke 20.

The frame 4 is constituted by a pipe. In other words, the frame 4 is hollow. The material of the pipe is a fiber-reinforced resin. The matrix resin of the fiber reinforced resin is a thermosetting resin. The thermosetting resin is typically an epoxy resin. The fibers of the fiber reinforced resin are typically carbon fibers. The fibers are filament fibers.

The grip 6 is formed by a tape wound around the shaft 16. The grip 6 suppresses a slip between a hand of a player and the tennis racket 2 when the tennis racket 2 is swung.

The string 10 is stretched on the head 12. The string 10 is stretched along the width direction X and the axial direction Y. Of the string 10, portions extending along the width direction X are referred to as transverse strings 10 a. Of the string 10, portions extending along the axial direction Y are referred to as longitudinal strings 10 b. The face 18 is formed by a plurality of transverse strings 10 a and a plurality of longitudinal strings 10 b. The face 18 generally extends along an X-Y plane.

FIG. 2 shows the string 10. The string 10 is of a monofilament type. Alternatively, the string 10 may be formed by twisting a plurality of filaments together. The string 10 may have a plurality of layers. The string 10 may have a coating on its surface.

Preferably, the material of the string 10 is a resin composition. Preferably, the base resin of the resin composition is a thermoplastic resin. The resin composition may contain various additive agents in addition to the base resin. Specific examples of the additive agents include an ultraviolet absorber, a softener, a plasticizer, and a colorant.

Specific examples of the thermoplastic resin suitable for use in the string 10 include polyester and nylon. The string 10 in which polyester or nylon is used has better durability than a natural gut string. Even when the string 10 hits a tennis ball repeatedly, breakage of the string 10 does not easily occur.

Hereinafter, a method of manufacturing the string 10 is described. In the manufacturing method, first, an untreated string is prepared. The untreated string means a string that has not been subjected to irradiation with radioactive rays, which will be described below.

A known method can be adopted for preparing the untreated string. Typically, a drawing process is adopted for preparing the untreated string. In the drawing process, the base resin is heated and melted, to which additive agents are added, and thereby a molten resin composition is obtained. The molten resin composition is cooled while being extruded from a spinneret, and thereby a base string is obtained. The base string is stretched, and thereby the untreated string is obtained.

Next, the untreated string is irradiated with radioactive rays. Examples of the radioactive rays include alpha rays, beta rays, gamma rays, and X-rays. Use of gamma rays is preferable since gamma rays have been used in industrial settings. Gamma rays have been put into practical use to kill pests and bacteria on food products after their packaging. Gamma rays have also been put into practical use to prevent sprouting of agricultural produce, such as potatoes. Gamma rays are generated due to a phenomenon in which when the energy level of atomic nuclei shifts from an unstable state to a stable state, the atomic nuclei release energy. Typically, cobalt-60 gamma rays are used. Immediately after the drawing process is completed, the untreated string may be irradiated with the radioactive rays on the drawing equipment. Alternatively, after the drawing process is completed, the untreated string that has been wound up may be irradiated with the radioactive rays. Further alternatively, the untreated string that has been cut to a predetermined size and packaged may be irradiated with the radioactive rays.

As a result of the irradiation with the radioactive rays, the untreated string is softened, and thereby the string 10 is obtained. It is considered that the reason for the untreated string to be softened is that resin crystals that are produced in the untreated string due to the drawing or the like disappear when the untreated string is irradiated with the radioactive rays. One untreated string may be irradiated with the radioactive rays a plurality of times repeatedly.

When a tensile load is applied to the string 10, the string 10 stretches sufficiently. When a tennis ball is hit with the racket 2 in which the string 10 is used, the face 18 deforms sufficiently. The player feels that the hitting feeling of the racket 2 is “soft”. The string 10 can contribute the hitting feeling of the racket 2. In addition, the string 10 can reduce damage to a joint of the player.

As previously mentioned, in light of durability, the base resin is preferably polyester. By irradiating the untreated string containing polyester with the radioactive rays, the hitting feeling of the string can be brought close to the hitting feeling of the string 10 in which nylon is used. The irradiation with the radioactive rays does not significantly impair the durability.

Alternatively, the untreated string whose base resin is nylon may be irradiated with the radioactive rays. As a result of the irradiation with the radioactive rays, the string 10 having a hitting feeling close to that of natural gut can be obtained. The irradiation with the radioactive rays does not significantly impair the durability.

In a case where the base resin is, for example, polyester, an elongation rate P1 of the string 10 when the string 10 is tensioned to a stress of 217 MPa is preferably higher than or equal to 9.25% but lower than or equal to 18.6%. The string 10 having the elongation rate P1 of higher than or equal to 9.25% has an excellent hitting feeling. In light of this, the elongation rate P1 is more preferably higher than or equal to 9.30%, and particularly preferably higher than or equal to 9.35%. The string 10 having the elongation rate P1 of lower than or equal to 18.6% has excellent durability. In light of this, the elongation rate P1 is more preferably lower than or equal to 17.0%, and particularly preferably lower than or equal to 15.0%.

In a case where the base resin is, for example, nylon, an elongation rate P2 of the string 10 when the string 10 is tensioned to a stress of 221 MPa is preferably higher than or equal to 19.3% but lower than or equal to 22.0%. The string 10 having the elongation rate P2 of higher than or equal to 19.3% has an excellent hitting feeling. In light of this, the elongation rate P2 is more preferably higher than or equal to 19.5%, and particularly preferably higher than or equal to 19.7%. The string 10 having the elongation rate P2 of lower than or equal to 22.0% has excellent durability. In light of this, the elongation rate P2 is more preferably lower than or equal to 21.5%, and particularly preferably lower than or equal to 21.2%.

According to research conducted by the inventor of the present invention, an average tensile force when a general string is stretched on a tennis racket is in a range from 217 MPa to 221 MPa. Therefore, in the present invention, the string 10 is evaluated based on the elongation rate P1 when the string 10 is tensioned to a stress of 217 MPa and the elongation rate P2 when the string 10 is tensioned to a stress of 221 MPa.

These elongation rates P1 and P2 are measured by a tensile test machine. The test machine includes a pair of chucks. The string 10 is wound around each chuck. Test conditions are as indicated below.

Test machine: SHIMADZU AG-Xplus 5 kN Chuck: SHIMADZU 5 kN air capstan type, 343-23325-30 Inter-chuck distance: 210 mm Inter-gauge mark distance: 150 mm Tensile speed: 1 mm/sec Plotting interval: 100 msec Ambient temperature: 25° C. The measurement is performed three times for each string 10, and the median value is used as a representative value.

The irradiation amount of the radioactive rays is preferably greater than or equal to 250 kGy but less than or equal to 2000 kGy. The string 10 having an excellent hitting feeling can be obtained by a manufacturing method in which the irradiation amount is greater than or equal to 250 kGy. In light of this, the irradiation amount is more preferably greater than or equal to 350 kGy, and particularly preferably greater than or equal to 400 kGy. The irradiation with the radioactive rays in an irradiation amount of less than or equal to 2000 kGy can be performed at low cost. In light of this, the irradiation amount is more preferably less than or equal to 1600 kGy, and particularly preferably less than or equal to 1400 kGy. In a case where one untreated string is irradiated with the radioactive rays a plurality of times repeatedly, the total irradiation amount is preferably within the above range.

EXAMPLES Experiment 1 Test Example 1

A commercially available string (Dunlop tennis string: trade name “Black Widow”) was prepared. The base resin of the string is polyester. The string was irradiated with cobalt-60 gamma rays. The irradiation amount was 500 kGy. Then, the string was further irradiated with cobalt-60 gamma rays in an irradiation amount of 500 kGy. That is, the total irradiation amount was 1000 kGy. The string was subjected to a tensile test. In the test, the elongation rate of the string was measured at loads of 40 lbs, 50 lbs, 60 lbs, and 70 lbs. The measurement results are shown in Table 1 below.

Test Example 2

A commercially available string I was prepared. The base resin of the string is polyester. The elongation rate of the string was measured in the same manner as in Test Example 1. The measurement results are shown in Table 2 below.

Test Example 3

A commercially available string II was prepared. The base resin of the string is polyester. The elongation rate of the string was measured in the same manner as in Test Example 1. The measurement results are shown in Table 2 below.

Test Example 4

A commercially available string III was prepared. The base resin of the string is nylon. The string was irradiated with cobalt-60 gamma rays a plurality of times. The irradiation amounts were 250 kGy, 500 kGy, and 1000 kGy, respectively. The string was subjected to a tensile test. In the test, the elongation rate of the string was measured at loads of 50 lbs, 60 lbs, and 70 lbs. The measurements results are shown in Table 3 below.

TABLE 1 Evaluation of Elongation Rate (%). Commercially Available Product Test Example 1 Gut Black Widow (Natural) Polyester Gamma Rays (kGy) 0 0 500 1000 40 lbs 10.72 4.1 4.8 5.2 50 lbs 14.82 5.8 7.3 7.6 60 lbs 18.79 7.57 9.79 11.81 70 lbs 22.72 9.6 13.8 22.7

TABLE 2 Evaluation of Elongation Rate (%). Test Example 2 Test Example 3 Commercially Commercially Available Available Product I Product II Polyester Polyester Gamma Rays (kGy) 0 500 1000 0 500 1000 40 lbs 4.37 5.12 4.86 4.89 4.75 4.75 50 lbs 6.3 8.4 10.4 7.1 7.1 7.6 60 lbs 8.1 10.8 13.3 9.2 9.4 10.1 70 lbs 7.7 10.4 12.8 11.1 11.6 12.5

TABLE 3 Evaluation of Elongation Rate (%). Test Example 4 Commercially Available Product III Nylon Gamma Rays (kGy) 0 250 500 1000 50 lbs 15.8 16.4 18.7 19.2 60 lbs 17.8 19.3 21.4 21.9 70 lbs 19.7 20.7 23.3 24.4

It is clear from Tables 1 to 3 that there is a tendency for the irradiation with the radioactive rays to increase the elongation rate of the string.

Experiment 2 Example 1

A commercially available string (the aforementioned string, i.e., trade name “Black Widow”) was prepared. The base resin of the string is polyester. The string was irradiated with cobalt-60 gamma rays. The irradiation amount was 250 kGy.

Examples 2 to 3 and Comparative Example 1

Strings of Examples 2 to 3 and Comparative Example 1 were obtained in the same manner as in Example 1, except that the radioactive ray irradiation amount was varied as shown in Table 4 below.

Examples 4 to 9 and Comparative Examples 2 and 3

Strings of Examples 4 to 9 and Comparative Examples 2 to 3 were obtained in the same manner as in Example 1, except that the starting raw material string and the radioactive ray irradiation amount were varied as shown in Tables 5 and 6 below.

Comparative Example 4

A commercially available natural gut string was prepared as Comparative Example 4.

Elongation Rate P1

The elongation rate P1 of each string was measured in the previously described manner. The measurement results are shown in Tables 4 to 6 below.

Hitting Feeling

A player used tennis rackets on which the respective strings were stretched, and evaluated the hitting feeling of each string on a scale of 1 to 5. The evaluation results are shown in Tables 4 to 6 below. The greater the numerical value, the better the hitting feeling.

Durability

The durability of each string was evaluated by a stress relaxation test machine on a scale of 1 to 5. Test conditions were as indicated below.

Mode: Tension Load: 245 N

Tensile speed: 1 mm/sec The evaluation results are shown in Tables 4 to 6 below. The greater the numerical value, the higher the durability.

Overall Evaluation

The sum of the evaluation value of the hitting feeling and the evaluation value of the durability was used as an overall evaluation value. The overall evaluation results are shown in Tables 4 to 6 below.

TABLE 4 Evaluation Results. Comparative Example Example Example Example 1 1 2 3 Untreated Black Black Black Black String Widow Widow Widow Widow Material Polyester Polyester Polyester Polyester Gamma 0 250 500 1000 Rays (kGy) P1 (%) 6.53 7.44 8.29 9.77 Hitting 1 3 4 4 Feeling Durability 5 5 5 4 Overall 6 8 9 8 Evaluation

TABLE 5 Evaluation Results. Comparative Example Example Example Example 2 4 5 6 Untreated Commer- Commer- Commer- Commer- String cially cially cially cially Available Available Available Available Product I Product I Product I Product I Material Polyester Polyester Polyester Polyester Gamma 0 250 500 1000 Rays (kGy) P1 (%) 9.19 9.26 9.45 10.12 Hitting 2 3 4 4 Feeling Durability 5 5 5 4 Overall 7 8 9 8 Evaluation

TABLE 6 Evaluation Results. Comparative Comparative Example Example Example Example Example 3 7 8 9 4 Untreated Commercially Commercially Commercially Commercially Commercially String Available Available Available Available Available Product II Product II Product II Product II Product Material Polyester Polyester Polyester Polyester Gut (Natural) Gamma 0 250 500 1000 0 Rays (kGy) P1 (%) 8.37 9.89 10.75 10.36 18.79 Hitting 2 3 4 4 5 Feeling Durability 5 5 5 4 2 Overall 7 8 9 8 7 Evaluation

Experiment 3 Example 10

A commercially available string (the aforementioned string III) was prepared. The base resin of the string is nylon. The string was irradiated with cobalt-60 gamma rays. The irradiation amount was 250 kGy.

Examples 11 to 12 and Comparative Example 5

Strings of Examples 11 to 12 and Comparative Example 5 were obtained in the same manner as in Example 10, except that the radioactive ray irradiation amount was varied as shown in Table 7 below.

Comparative Example 6

A commercially available natural gut string was prepared as Comparative Example 6.

Elongation Rate P2

The elongation rate P2 of each string was measured in the previously described manner. The measurement results are shown in Table 7 below.

Hitting Feeling

A player used tennis rackets on which the respective strings were stretched, and evaluated the hitting feeling of each string on a scale of 1 to 5. The measurement results are shown in Table 7 below. The greater the numerical value, the better the hitting feeling.

Durability

The durability of each string was evaluated by a stress relaxation test machine on a scale of 1 to 5. Test conditions were as indicated below.

Mode: tension

Load: 245 N

Tensile speed: 1 mm/sec The measurement results are shown in Table 7 below. The greater the numerical value, the higher the durability.

Overall Evaluation

The sum of the evaluation value of the hitting feeling and the evaluation value of the durability was used as an overall evaluation value. The overall evaluation results are shown in Table 7 below.

TABLE 7 Evaluation Results. Comparative Comparative Example Example Example Example Example 5 10 11 12 6 Untreated Commercially Commercially Commercially Commercially Commercially String Available Available Available Available Available Product III Product III Product III Product III Product Material Nylon Nylon Nylon Nylon Gut (Natural) Gamma 0 250 500 1000 0 Rays (kGy) P2 (%) 17.8 19.3 21.4 21.9 19.1 Hitting 3 4 5 5 5 Feeling Durability 4 4 4 3 2 Overall 7 8 9 8 7 Evaluation

As shown in Tables 4 to 7, the string according to each Example has an excellent hitting feeling-durability balance. These evaluation results clearly indicate the superiority of the present invention.

The string according to the present invention is suitable for use in various sports, such as tennis, soft tennis, squash, and badminton.

The foregoing description is in all aspects illustrative, and various modifications can be made without departing from the essential features of the present invention. 

What is claimed is:
 1. A method of manufacturing a string for use in a racket, the method comprising: (A) preparing an untreated string; and (B) irradiating the untreated string with radioactive rays.
 2. The method according to claim 1, wherein the step (B) includes irradiating the untreated string with gamma rays.
 3. The method according to claim 1, wherein the step (B) includes irradiating the untreated string with the radioactive rays in an irradiation amount of greater than or equal to 250 kGy but less than or equal to 2000 kGy.
 4. The method according to claim 1, wherein a material of the untreated string prepared in the step (A) is a resin composition whose base material is a thermoplastic resin.
 5. The method according to claim 4, wherein the thermoplastic resin is polyester or nylon.
 6. A string for use in a racket, wherein an elongation rate P1 of the string when the string is tensioned to a stress of 217 MPa is higher than or equal to 9.25% but lower than or equal to 18.6%.
 7. The string according to claim 6, wherein a material of the string is a resin composition whose base material is a thermoplastic resin, and the thermoplastic resin is polyester.
 8. A string for use in a racket, wherein an elongation rate P2 of the string when the string is tensioned to a stress of 221 MPa is higher than or equal to 19.3% but lower than or equal to 22.0%.
 9. The string according to claim 8, wherein a material of the string is a resin composition whose base material is a thermoplastic resin, and the thermoplastic resin is nylon.
 10. A racket comprising a frame and a string, wherein an elongation rate P1 of the string when the string is tensioned to a stress of 217 MPa is higher than or equal to 9.25% but lower than or equal to 18.6%.
 11. A racket comprising a frame and a string, wherein an elongation rate P2 of the string when the string is tensioned to a stress of 221 MPa is higher than or equal to 19.3% but lower than or equal to 22.0%. 